Mortality from invasive breast cancer has remained high, despite advances in early detection, diagnosis and treatment. Identification of women at high risk could result in targeted prevention efforts. Women who have undergone previous breast biopsy but were not positive for cancer at that time, are believed to be at elevated risk for future breast cancer, particularly if certain types of benign conditions were present at time of biopsy. This project proposes to apply new techniques to analyze biopsied material and identify potential biologic markers, thus sharpening criteria for determination of increased risk of invasive breast cancer. Biopsied material characteristics, individually and in combination, will be evaluated as markers among women who have undergone a previous breast biopsy. These characteristics include measures of cellular characteristics (histologic type, S-phase fraction, and ploidy) and control mechanisms (receptors, proliferating cell nuclear antigen (PCNA), oncogene expression, and cathepsin-D). Ploidy and S-phase fraction will be assessed by flow cytometry on tissue samples collected at the time of initial breast biopsy. Cathepsin-D, PCNA, cerbB-2 expression, and estrogen and progesterone receptors will be assessed by immunocytochemical techniques. The population consists of women aged 40 and older enrolled in the Breast Cancer Screening Program (BCSP) of a large HMO. Since 1985, 87,892 women have enrolled in the BCSP and have completed a comprehensive questionnaire regarding breast cancer risk factor information. Attention is restricted to 2,734 women who had a non-positive breast biopsy performed since 1984. An estimated 67 women were subsequently diagnosed with invasive disease. Biologic markers for this group and an equal number of control women will be compared in a case-cohort study. This design emulates a cohort study but reduces the number of tissue samples to be evaluated. The independent contribution of each marker will be tested after controlling for potential confounders, such as age, family history, parity, and hormonal information. The impact of type of benign breast disease and screening history will also be evaluated. It is hypothesized that the rate of cell growth and presence or absence of control mechanisms will augment risk information conveyed by benign breast disease type and will enhance estimation of risk of future invasive breast cancer.